Switching system for the acquisition of seismic data

ABSTRACT

A seismic recording system comprises a multichannel recorder connected to a cable formed from a plurality of signal conductors. Each of the signal conductors is connected to a different channel of the recorder. The cable has a plurality of takeouts located at spaced distances along its length. Each of the takeouts is connected to a different signal conductor of the cable. The system further includes a first conduit operably connected to one of the takeouts. A plurality of remotely operable switch means are located along the length of the first conduit for selectively interconnecting the first conduit and a plurality of detector conduits upon receipt of a remotely generated signal. Each detector conduit has at least one geophone located along its length and operably connected thereto. A control means is provided for transmitting a signal to a selected switch means for interconnecting its associated detector conduit and geophones with the recorder.

FIELD OF THE INVENTION

The invention relates to seismic systems for the systematic surveying orexploring of extended geographical areas. The invention particularlyrelates to obtaining seismic data for use in locating subsurface stratawhich might contain valuable minerals or hydrocarbons.

BACKGROUND OF THE INVENTION

In geophysical prospecting, seismic operations are frequently performedto generate and collect information indicative of the physicalproperties of subsurface formations. A conventional seismic landoperation includes placing a seismic cable along the surface of theearth. Usually, the cable extends in a straight line and contains aplurality of wire pairs. A typical cable will contain from 48 to about120 wire pairs. At regular intervals along the cable, there are providedelectrical connectors generally referred to as "takeouts." Each takeoutis electrically connected to a separate pair of wires in the seismiccable. Typically, the takeouts are located at intervals of 50 to 200feet along the length of the cable.

An array of geophones is attached to each takeout of the seismic cable.The array of geophones may consist of from one to fifty or moreindividual geophones. Each geophone is typically affixed to the surfaceof the earth by means of a spike so that, as the earth moves, thegeophone moves as well. The geophones in any individual array areelectrically interconnected so that the output signal from the group isa measure of the average motion sensed by all the geophones in thatspecific array.

In a typical seismic survey for a large geographical area, the area iscovered by a plurality of survey lines. Seismic profiles are thenrecorded along these survey lines. On land, a seismic cable, typicallyapproximately two miles in length and formed from a series of identicalsections, is laid on the ground along a survey line. At each takeout,there is connected a two-wire conduit to electrically interconnect, forexample, about 30 geophones into a single group or array. Each geophonegroup is located in the general vicinity of the take out of the seismiccable to which it is attached.

A seismic signal is generated in the vicinity of the geophone groups.Typical seismic sources include explosive charges, weight drops, orother impact sources which impart a sudden impulse of energy to thesurface of the earth in the survey area. Another type of seismic sourceis a vibrator which imparts a vibratory sweep of energy to the earth'ssurface for a period of time from a few seconds to about 20 seconds.Typically, the seismic signal is imparted into the earth at regularlyspaced intervals along a portion of the length of the seismic cable.

One end of the seismic cable is connected to a seismic recorder, whichtypically includes a magnetic tape recorder. The seismic waves from eachgroup of geophones are recorded on the magnetic tape in digital form. Amultiple position switch, referred to as a roll-along switch, in arecording truck is advanced to a new position, thereby advancing theportion of the cable connected to the data recording equipment. Afterthe desired data have been recorded, one or more of the cable sectionsare disconnected from one end of the two mile length of cable andreconnected to the other end. A new data recording cycle is thenundertaken. After recordings have been taken along the full length ofone survey line, the cable, geophones and recorder are moved to the nextsurvey line and the process repeated until the entire geographical areahas been covered.

The foregoing type of data recording is generally referred to as thereflection method in which the seismic waves or impulses are generatedat or near the earth's surface and these waves are reflected fromsubsurface acoustic impedance boundaries and detected by the array ofseismic detectors located at the surface. Detector groups ofconsiderable length are used preferentially in reflection seismicexploration in order to discriminate between signals and unwanted noise.

A disadvantage of having long lengths of detectors is the attenuation ofhigh frequency signals. Attempts have been made to improve the highfrequency response of seismic rays by using very short (a smaller numberof detectors) arrays. The most common result is a considerabledegradation of data quality owing to the decrease in signal-to-noiseratio.

It has been considered that it might be possible to restore thesignal-to-noise ratio to a value comparable to that of arrays in commonuse by greatly increasing the number of arrays, in effect retaining thenumber of individual detectors in common use but subdividing them intomany more but shorter arrays. A difficulty resulting example, if afull-length, 2-mile seismic cable is to be employed and if the sensordensity (number of sensors) is to remain unchanged, the number of signalchannels which must be connected to the recorder is increased by anorder of magnitude as the group length is reduced. This would mean about10 times as many conductor wire pairs would have to be added to thecable if the group lengths were to be significantly reduced. Such anincrease in the number of conductor wire pairs would greatly increasethe weight and the bulk of the cable sections and decrease theirflexibility to unacceptable levels.

SUMMARY OF THE INVENTION

This invention provides a seismic testing system capable of recordingdata from a wider area than was heretofore possible without the need foradded wires or signal conductors in the cable. It also permits the useof shorter groups of sensors in a long array, also without the need forany added wire pairs in the cable. Thus, the present invention can alsobe utilized to maintain a high signal-to-noise ratio while at the sametime enhancing the high frequency response.

Broadly, the system comprises a multichannel recorder and a length ofcable comprising a plurality of signal conductors, each of the signalconductors being connected to a different channel of the recorder. Thecable is provided with a plurality of takeouts located at spaceddistances along the length of the cable. Each of the takeouts isconnected to a different signal conductor. The system further includes aplurality of primary conduits, each of which has one end connected to adifferent takeout, and a plurality of switch means, operably connectedto each of said primary conduits. The switch means are located at spacedintervals along the length of the primary conduit. The system alsoincludes a number of secondary detector conduits, each of which has anend operably connected to a different one of said switches. Each switchis operable between a position in which the primary and secondaryconduits are electrically interconnected, and another position in whichthe primary and secondary conduits are electrically isolated from oneanother. An array of seismic detectors is located along the length ofeach of the secondary conduits and operably connected thereto. Thesystem further includes a control means for transmitting a signal,through selected signal conductors, to a selected switch means forinterconnecting an array of seismic detectors with said recorder.

Advantageously, each switch means comprises an integrated circuit whichis separately addressable by an input binary code. The integratedcircuit is electrically connected to a solid state switch which isoperated by an electrical impulse from the integrated circuit. The solidstate switch is energizable between two positions. In one position itinterconnects the primary detector conduit with the array of geophoneslocated on the associated secondary conduit. In the other position itisolates the array of geophones on the secondary conductor from theprimary detector conduit and interconnects the primary detector conduit.

The switch means also may include means for maintaining the selectedsolid state switch(s) in a selected position. Typically this isaccomplished with a battery. It is preferred to use a solid state switchof the locking type which does not require a voltage potential tomaintain it in either position.

BRIEF DESCRIPTION OF THE DRAWINGS

The use and advantages of the invention will be more apparent from aconsideration of the following detailed description and the drawings inwhich:

FIG. 1 is a diagram of a seismic system as hereinbefore practiced;

FIG. 2 is a diagram of a seismic system utilizing the present invention;and

FIG. 3 is a schematic of a switch for use in the present invention.

DETAILED DESCRIPTION OF THE INVENTION

It is believed that a review of a prior art seismic recording systemwill be beneficial to a better understanding of the present inventionand the advantages obtained therefrom. Referring to FIG. 1, a typicalprior art seismic system utilizing a reflective method comprises arecorder 12 and a seismic conduit 14 which extends along the surface ofthe earth, typically along a survey line. Cable 14 generally extends adistance of approximately two miles and is made up of individual cablesections, each section having a length of about 300 to 400 feet. It willbe appreciated that in many instances the cable must be laid acrossterrain not accessible by vehicle. Thus, the cable preferably is inlengths having a weight which can be carried by one man.

Located along the length of cable 14 are a plurality of electricalconnectors or takeouts T. The number of takeouts T_(n) is equal to thenumber of wire pairs making up cable 14. Typically, the takeouts T arelocated at substantially uniform intervals of about 50 to 100 feet. Ateach takeout T, there is connected thereto one or more conduits 16. Asdepicted, at T₁, there is a conduit 16 (typically comprised of twowires) which extends substantially parallel to cable 14. Atsubstantially regular intervals along the length of conduit 16 are aplurality of individual geophones 18 which are electrically connected tothe two wires of conduit 16.

When a single linearly extending conduit is utilized, it typically willhave a length of about 30 to 300 feet and be provided with 1 to about 50geophones at spaced intervals along that length. Various otherconfigurations are utilized for certain special applications and terrainconditions. The configuration shown at T₁ is one commonly used.

To initiate the collection of seismic data, a seismic source signal istransmitted into the earth, for example, at VP. The source signaltravels across the surface of the earth and is sensed by geophones 18.In addition, the same signal radiates downwardly through the earth andis reflected back to the geophones from subsurface boundary layers. Theformer is undesirable noise and the latter is the source of the signalof interest. The seismic source may be stationary or it may be movedalong a portion of the length of cable 14.

It will be appreciated that, while the array of geophones is shown asextending in only one direction, in many instances it will projectlinearly in both directions, and in some instances the number andarrangement of geophones at each takeout T may vary. The electricalsignals generated by geophones 18 and transmitted to any given takeoutrepresent a weighted average of the signals from all the geophones inthat group. That average signal passes through an individual wire pairof cable 14 to recorder 12.

It also will be appreciated by those skilled in the art that the signalspassing through each wire pair of cable 14 are not directly input to aseparate channel of recorder 12. Generally the signals are introducedinto a multiplexer which scans each of the wire pairs in sequence for afinite length of time. Typically, the multiplexer will processapproximately 1,000 samples per second from cable 14. The individualsamples can be recorded in analog form. However, they are not amenableto computer processing in such form. Thus, they preferably are passedthrough an analog to digital convertor prior to being recorded on theirrespective channel of the magnetic tape recorder. In addition,amplifiers also may be provided to increase the amplitude of signalsfrom the respective takeouts.

Once a recording has been completed, a number of sections forming cable14 are removed from one end and reconnected to the other end, along withtheir conduits 16 and geophones 18, and the recording cycle repeateduntil the linear length of the survey line has been recorded.Thereafter, the entire length of cable 14, conduits 16, geophones 18,and recorder 12 are moved to the next survey line and recording startedagain. Obviously, the movement of approximately two miles of heavy cableis a labor-intensive and expensive task. The use of a long array ofgeophones such as shown at T₁ provides a high signal-to-noise ratio butdetracts from its ability to receive and transmit high frequencyimpulses.

Referring now to FIG. 2, therein is shown a seismic system arranged inaccordance with the present invention (like numbers in FIG. 1 referringto like parts in FIG. 2). The seismic system shown in FIG. 2, like thatshown in FIG. 1, includes a recorder 12, and a seismic cable 14comprised of a plurality of signal conductors and is provided with anumber of takeouts T_(n). The number of takeouts may be from as few as100 to as many as 1000 or more with a fiber optic system. A typicalnumber of takeouts for a wire pair system is about 100 to 200. A conduit16 is attached to a signal conductor of each takeout. Typically, eachconduit will have a length of from 200 feet to a mile, but in someapplications they may be substantially longer. The present invention isapplicable to systems using cables comprised of wire pairs or opticalfibers for signal conductors. For convenience and purposes of comparisonwith the previously described system, the invention will be describedwith reference to a cable formed from a plurality of wire pairs.

For clarity, only three connections to takeouts T are shown. In contrastto the prior art, conduit 16 extends through a plurality of switch means22, rather than directly to geophones 18. The purpose of switch means 22is to interconnect a takeout via conduit 16 with any one or more of aplurality of arrays of geophones via a plurality of conduits 20. In theinterest of clarity, only three arrays of geophones are shown attachedto each of the two conduits 16 at T₁ and T₂. However, in actualpractice, it is anticipated that a sufficient number of switch means 22would be provided for attachment of from 5 to 25 and preferably from 10to 20 separate arrays of geophones. Switch means 22 provides forselectively interconnecting a takeout to the conduit 20 associated withthat particular switch (along with its associated array of geophones).The purpose for such selectivity will be described more fully later.

Switch means 22 could comprise conventional mechanical switchesactivated by solenoids or the like. However, in the interest of rapidswitching speed and reducing the electrical power requirements of theswitch, it is preferred that switch means 22 be formed from solid statecircuitry.

Referring to FIG. 3, for example, switch means 22 could comprise anintegrated circuit 28 connected to conduit 16 by a wire pair 30. Uponreceipt of an appropriate binary code through wire pair 30, integratedcircuit 28 would transmit a signal through a conductor 32 to a solidstate switching device 34. The solid state switching device would, inturn, interconnect conduit 20 and its array of geophones with conduit 16and its associated takeout. Thus, utilizing an appropriate combinationof binary signals, it is possible to interconnect an array or pluralityof arrays with the takeout by activation of selected switches. It willbe noted that the integrated circuit is separately connected to conduit16 such that it is always addressable by a signal through the wire pairof conduit 16 to which it is connected.

As those skilled in the art will appreciate, most solid state switchesrequire that a voltage potential be applied to them to maintain theswitch in a closed (conducting) position. This is readily accomplishedwith a battery. Since the power requirements of the solid state devicesis low, a small 9 volt battery will generally suffice. In some instancesit may be advantageous to use a rechargeable battery. The wire pair ofconduit 16 associated with the switch then could be used to supplycurrent to and recharge the battery when the the system is not recordingdata.

Referring back to FIG. 2, the system also includes a controller 24 whichis electrically interconnected via a cable 26 to all the wire pairsbeing utilized in cable 14 for switch means 22. The purpose ofcontroller 24 is to send a signal through the wire pair associated withany specific switch means 22 and thereby cause switch means 22 tointerconnect designated arrays of geophones with its associated conduit16.

Since both control signals from controller 24 and input signals fromgeophones 18 are conveyed over the same wire pair, some means fordifferentiating between the two signals is required to prevent switchmeans 22 from interpreting a geophone signal as a control signal. Apreferred way of accomplishing this, in accordance with the presentinvention, is to use a relatively high amplitude signal for control. Theinput signal from the geophones will have a relatively low amplitude.Thus, the controller and switch means are readily designed tocooperatively operate only at a higher voltage amplitude than will beproduced in normal operation by the geophones.

Preferably, controller 24 is a computer which can be utilized to eithermanually generate the desired digital signal or programmed toautomatically generate a series of signals for interconnecting selectedarrays of geophones with cable 14. Specifically, the computer can beprogrammed for sampling of the inputs to the various takeouts and alsosample individual arrays of geophones connected through switch means 22.Thus the present invention provides a degree of versatility far beyondthat possible with the prior art systems.

The practice of the present invention will be described by comparisonwith the operation of a prior art seismic system. It is believed suchcomparison will more clearly illustrate the practice and advantages ofthe present invention.

Referring to FIG. 1, for purposes of illustration, assume cable 14 has alength of 10,000 feet and is provided with 100 takeouts. Each takeouthas a general configuration as shown at T₁, namely, the takeout isprovided with an array comprising a linearly extending cable 16 having alength of 150 feet. Cable 16 is provided with 30 geophones 18 at evenlyspaced intervals. The arrays of geophones overlap one another. Thepurpose of the overlap of arrays is to enhance the signal to noise ratioof the signals received.

Such a system would have a relatively high signal-to-noise ratio byvirtue of the length of the arrays. However, the length would be such asto attenuate high frequency signals which could be of value in assessingsubterranean geological formations. Any changes in the number ofgeophones, length of the array, or arrangement of the array wouldnecessitate manually connecting, disconnecting, or moving individualgeophones. Clearly, this system provides little flexibility once it isinstalled and involves a substantial amount of physical labor moving itfrom one location to another.

Referring now to FIG. 2, utilizing a cable 14 having the same length andan equal number of takeouts, a controller and a plurality of switchmeans 22 along the length of conduit 16 as hereinabove described, ateach switch means there are several linearly extending arrays ofgeophones which may be arranged and utilized in several ways. Forexample, as depicted, it is possible to utilize three or more arrays ofvarious lengths. The arrays from each switch means 22 could stilloverlap as in FIG. 1

Group A would have a length of 50 feet and contain an array of 10geophones along its entire length. Group B would have a length of 100feet and contain an array of 10 geophones extending 50 feet beyond GroupA. Group C would then have a length of 150 feet with an array of 10geophones extending along the last 50 feet of its conduit 20. Such anarrangement of short, staggered arrays will provide the benefit of ahigh signal-to-noise ratio while concurrently providing an enhancedsensitivity to higher frequencies than would be obtainable with thesystem shown in FIG. 1.

In addition, the system depicted in FIG. 2 could be readily tuned onlocation for obtaining best results. As an example, a test charge couldbe detonated to send a seismic impulse into the earth. Data samplescould then be taken from each array and various combinations of thearrays to determine which array or combination yielded the best data. Insuch instance it might be found that the best signal-to-noise ratio andfrequency response is obtained at one take out by interconnecting T_(n)and all the arrays of seismic geophones, whereas at another take out thebest signal-to-noise ratio and frequency response might be obtained fromone array to the exclusion of the others. This optimum arrangement canreadily be determined by varying the arrays interconnected through thevarious takeouts prior to the actual acquisition of seismic data.

To obtain optimum recording results it may be preferred to use an arrayhaving a length greater than the wave length of the seismic wave havingthe lowest undesirable frequency. In addition, the distance betweenadjacent geophones should be less than the wave length of the seismicwave having the highest undesirable frequency (shortest wave length).Generally, this knowledge is not known a priori and may vary atdifferent locations within the geographical area being surveyed. Withthe present invention, arrays of different length and different spacingbetween geophones may be used at each switch means, and the optimumarray determined experimentally by firing a test shot.

It also must be appreciated that while only 3 arrays are shown attachedto conduit 16 via switch means 22, it is anticipated that theretypically would be from 10 to 15, thus providing an even greaterdiversity. Obviously, combinations of the foregoing techniques may beutilized for each of switch means 22. By combining the previouslydescribed advantage through use of a greater number of arrays connectedto conduit 16 via switch means 22, it is possible to both decrease thenumber of lateral moves required and enhance frequency response whilemaintaining a high signal-to-noise ratio. These benefits and advantagesare not suggested or even possible with the system such as shown inFIG. 1. Numerous other benefits and advantages obtained with the presentinvention will be readily apparent to those skilled in the art.

For example, one advantage of the system of the present invention isshown at T₃. Each array I of geophones is connected to a switch means 22and laterally displaced from cable 14. This type of grouping permitshigh density recording of the immediate and laterally extending areas.Such high density recording would not be feasible with a prior artsystem such as is shown in FIG. 1. The time and labor involved in movingthe prior art system laterally in small increments to accomplish thesame high density recording would be cost prohibitive.

Another advantage of the present invention is that the number ofrecorder channels required is substantially reduced. Through the use ofswitches as hereinbefore described far more arrays of geophones may bemonitored through, for example, a 120 channel recorder than wouldheretofore have been possible without physically disconnectingindividual arrays of geophones. With the present switching arrangementit is possible to monitor in excess of at least twice as many arrays ofdetectors. Indeed, the present invention makes it possible to monitorfrom 3 to 30 or more times as many arrays than would be possible withthe same recorder using prior art techniques.

It is emphasized that the foregoing description represents the preferredembodiments of the invention, and various alternative circuits orcomponents may be utilized in the system disclosed herein withoutdeparting from the spirit of the invention. Thus, for example, differentarrangements of arrays of geophones and other solid state devices may beemployed. Similarly, other data processing equipment may be employed toutilize the data collected in accordance with the present invention.Accordingly, the scope of the invention is to be determined by thefollowing claims and equivalences to which they are entitled.

What is claimed:
 1. A system comprised of:a multichannel recorder; alength of cable comprising a plurality of signal conductors, each of thesignal conductors being connected to a different channel of therecorder, the cable having a plurality of takeouts located at spaceddistances along the length of the cable and each of the takeouts beingconnected to a different signal conductor; a first conduit operablyconnected to one of said takeouts; a plurality of switch means dispersedalong said first conduit and operably connected thereto; a number ofdetector conduits, each of which has an end operably connected to adifferent one of said switch means; at least one seismic detectorlocated along the length of each of said detector conduits and operablyconnected thereto; and control means for transmitting a signal, throughselected signal conductors of said cable, to said switch means forelectrically interconnecting one or more selected detector conduits withsaid recorder.
 2. The system of claim 1 wherein said switch meanscomprises an integrated circuit having an outlet operably connected to asolid state switching device and each of said switch means beingseparately addressable by a signal.
 3. The switch means of claim 2wherein said signal is a binary electrical signal.
 4. The system ofclaim 3 wherein said control means comprises a computer means fortransmitting said binary electrical signal.
 5. The system of claim 1wherein said signal is a digital signal.
 6. The system of claim 1wherein said seismic detectors are geophones.
 7. The system of claim 1wherein said group of seismic detectors comprises from 1 to about 50seismic detectors.
 8. The system of claim 1 wherein from 2 to 50 switchmeans are provided along each of said first conduits.
 9. The system ofclaim 1 wherein the cable has from 100 to 1000 takeouts located atevenly spaced distances along the length of the cable.
 10. A seismicrecording system comprising:a multichannel magnetic tape recorder; acable comprising a plurality of signal conductors, each of the signalconductors being connected to a different channel of the recorder, thecable having a plurality of takeouts located at uniformly spaceddistances along the length of the cable and each of the takeouts beingconnected to a different signal conductor; a plurality of conduits, eachof which has an end operably connected to a different one of saidtakeouts; a plurality of switch means dispersed along the length of eachof said conduits and operably connected thereto; a plurality of detectorconduits, each of which has an end connected to a different one of saidswitch means; an array of seismic detectors located along the length ofeach of said detector conduits and operably connected thereto; andcontrol means for transmitting a digital signal through selected signalconductors of said cable to said switch means for electricallyinterconnecting selected detector conduits and arrays of seismicdetectors with said multichannel recorder.
 11. The system of claim 10wherein each of said signal conductors comprises a wire pair.
 12. Thesystem of claim 11 wherein said switch means comprises an integratedcircuit having an outlet in electrical communication with a solid stateswitching device and each of said switch means being separatelyaddressable by a signal.
 13. The system of claim 12 wherein said signalis a binary electrical signal.
 14. The system of claim 12 wherein saidcontrol means comprises a computer means for transmitting said binaryelectrical signal.
 15. The system of claim 13 wherein said seismicdetectors are geophones.
 16. The system of claim 13 wherein said groupof seismic detectors comprises from 1 to about 50 seismic geophones. 17.The system of claim 14 wherein from 1 to 50 switch means are providedfor each of said conduits.
 18. The system of claim 16 wherein the cablehas from 100 to 200 takeouts located at evenly spaced distances alongthe length of the cable.
 19. The system of claim 17 wherein each of thedetector conduits has a length of from 200 to 300 feet and the geophonesare located at substantially uniform intervals along its length.
 20. Thesystem of claim 16 wherein each of said conduits has from 5 to 25 switchmeans.
 21. The system of claim 14 wherein each of said conduits has from10 to 20 switch means.